Traditionally, chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) type refrigerants, such as CFC-11 (trichloromonofluoromethane), CFC-12 (dichlorodifluoromethane) and HCFC-22 (monochlorodifluoromethane) among others, have been used as refrigerants in refrigerators, air conditioners, chillers, commercial buildings and other appliances. These chlorine-based refrigerants are believed to destroy the ozone layer and therefore their use is to be gradually eliminated by 1996, under a recent protocol signed in Montreal, Canada by representatives of 167 countries of the world.
Chlorine-free hydrogen-containing halocarbons have already been introduced to replace CFC- and HCFC-type refrigerants. Hydrofluorocarbons (HFC), such as HFC-134 (1,1,2,2-tetrafluoroethane) and HFC-134a (1,1,1,2-tetrafluoroethane), are considered to be direct replacements for CFC-12 (also known as R-12) refrigerant. The cooling (thermodynamic) properties of HFC-134a are similar to those of the R-12 product in many applications and HFC-134a appears to have emerged as the currently preferred HFC refrigerant.
Historically, mineral oils, particularly naphthenic mineral oils, and alkylbenzenes, have been used as lubricants with the CFC-type refrigerants. However, the mineral oils have exhibited poor miscibility with HFC-type refrigerants: the resulting HFC/mineral oil mixture has been found to separate into two layers at ambient temperature. This results in the oil clogging in the cold temperature (evaporators) areas, thus restricting the refrigerant flow and causing poor oil return to the compressor, and it results in reduced efficiency. The lack of an effective lubricant to the compressor can also cause bearing seizure, and eventually compressor breakdown will occur.
Synthetic oils, such as polyalkylene glycol- and polyol ester-type refrigeration oils, have heretofore been introduced as lubricants for HFC-based systems. They have excellent miscibility with HFC-134a. See, for example, U.S. Pat. Nos. 4,948,525 and 4,755,316, which are hereby incorporated herein by reference in their entirety. These synthetic oils perform well in lubricating the compressor bearings.
However, in order to convert a CFC-based system to an HFC-based system, it is necessary to replace the mineral oil used as the lubricant in the CFC-based system with an HFC-134a-compatible lubricant such as those mentioned previously. To remove the mineral oil from the system, several flushes (say, a minimum of two to four) are required with the synthetic oils before a reasonably low and acceptable level (say, less than about 5%) of mineral oil can be achieved. For large systems, even more than four flushes can be required to reduce the mineral oil content to acceptable levels. This retrofitting procedure is thus quite expensive to carry out, since the useful synthetic oils, such as polyol esters, are relatively costly. After the synthetic oils have replaced the mineral oil in the system, the CFC is then removed and replaced with the HFC.
Until the present invention, the only lubricants reported to have been used in flushing mineral oils from CFC/mineral oil systems were polyglycols, hindered polyol esters, branched polyol esters or mixtures thereof. These prior lubricants are not only expensive but do not possess the inherent lubricity and higher solvating properties required to thoroughly cleanse the refrigeration system of mineral oil and chlorinated hydrocarbons. In addition, because of the hygroscopic nature of polyglycol and polyol esters, they disadvantageously tend to pick up water during a retrofitting operation which requires more than about three flushes to remove an adequate amount of the mineral oil from the system. It was also found that in the presence of chlorinated refrigerants, the prior art compositions hydrolyze even more, forming acids which can cause corrosion problems in compressors. To overcome these problems, additives (such as phenolic or amine antioxidants) are sometimes used, but the additives may not be compatible with the refrigerants. As a result, the additives can form a precipitate and cause circuit clogging and inefficient cooling.
Chlorinated solvents have also been used previously to flush mineral oils from compressors when the compressors have been retrofitted from CFC-12 to HFC-134a. Such chlorinated solvents have included CFC-11 and CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane). However, these are solvents, not lubricants, are toxic and are not chlorine-free.
Other prior art systems used to flush mineral oils from compressors have included mixtures of terpene hydrocarbon solvents and terpene alcohol compositions. See, for example, U.S. Pat. No. 5,174,906, which is hereby incorporated herein by reference in its entirety. These systems are very expensive to use and are not useful as lubricants.
Polyol ester-based compositions which have been used to flush mineral oils from CFC-based systems are commercially available from a number of manufacturers. See, for example, U.S. Pat. No. 5,021,179, which is hereby incorporated herein by reference in its entirety.
None of these methods of flushing mineral oils from a CFC-based refrigeration system has been found to be entirely satisfactory and relatively inexpensive.